In power transmission gear arrangements, a number of gear ratios are used to provide more efficient employment of a prime mover having a limited operating speed range.
The ratios are interchanged by engaging one friction device, such as a clutch, and disengaging another friction device. During a portion of the interchange period, both friction devices are at least partially engaged. The off-going device is held fully engaged until the on-coming device has a predetermined torque capacity.
The point at which the off-going device should begin to release is difficult to determine. Some prior art control systems have used the torque level at the output shaft as exemplified by stress therein.
Still other systems use a time phased control which is adjusted by the harshness or softness of the previous shift for a given set of input parameters. The most popular method of controlling upshift timing appears to be the use of a one-way torque transmitting device, either alone or in series, with a friction device.